breeding cats from the same litter

Question
my cat is pregnant from her brother is there going to be any side effects to the kittens from this?

Answer
This may occur occasionally due to opposite sex kittens being left together without neutering for too long.  Whether they will have any health repercussions depends on too many factors in their genetic makeup to determine easily.  If their parents and grandparents were unrelated and very strong with no genetic difficulties, then the chances for healthy kitten are good. But there is no way at present to be certain. So cross your fingers and alter your cats in future before this happens again.
Best of luck,

Regards,
Barb